The invention relates to optical imaging systems and methods, and more particularly to fluorescence imaging of light scattering media.
In vivo imaging of biological tissues facilitates early detection of disease, thereby providing an opportunity for reliable and pro-active diagnosis of diseased tissues. Fluorescence imaging is an example of a powerful non-invasive imaging technique that has been used in various applications in biological sciences. For example, fluorescence imaging is applied in fields such as genetic sequencing, biomedical diagnostics, and flow cytometry. Typically, fluorescence imaging systems include a light source which illuminates the subject to be imaged. The tissue inside the subject fluoresces either endogenously or exogenously in response to the excitation illumination, and this resulting emission is imaged to obtain information about the interior composition of the subject.
Fluorescence imaging is generally hampered by poor signal-to-noise ratio of fluorescent targets located within a subject. Much of this noise is caused by reflection of the excitation light from the surface, and by strong fluorescence signals emitted from points near the surface of the subject. Fluorescence imaging may be of different types, such as continuous wave, frequency domain, or time domain, and with each method the illumination and detection schemes are typically (a) point source illumination and point detection, or (b) planar illumination and full-field detection.
The point source illumination and point detection technique employs single pixel scanning for greater sensitivity, but this method may be very slow in generating a high-resolution image of the subject. In planar illumination with full-field detection, the entire area of the subject is illuminated and imaged, and while this method is capable of rapidly generating high-resolution images, it has poor sensitivity due to the low signal to noise ratio. The reflection of the excitation source increases the noise, and the limited amount of power that can be applied to the subject in full-field illumination mode limits the possible detected fluorescence signal in an absorbing medium such as tissue. Both the increased noise and limited signal contribute to a lower signal-to-noise ratio. Continuous wave planar illumination full-field imaging is also limited in its ability to determine the depth of the target.
Accordingly, there is a need for imaging systems and methods that can provide a high resolution, high sensitivity image in a shorter period of time.